Various applications require circuits to be powered down under certain conditions. Such conditions often include those times when the circuits are no longer in use. For example, consider an automotive application which requires a battery to provide a power to a circuit and where excessive current drain from the battery is a concern. If the power provided to the circuit is not inactivated when the circuit is not being used, the battery may require recharging or even replacement before the automobile may again be used.
This problem has been addressed in the past by inactivating all power to the circuit except for a minimal amount of power which is provided to a power activating switch. The power activating switch monitors externally provided signals (wake signals) which indicate when the power supply should provide power to the circuit. In response to this wake indication, power is provided to the circuit until the wake signals indicate that the power supply should be disabled, at which time the power activation circuit disables the power supply.
This approach is useful in that it provides an immediate battery saving function when the circuit is not being used. The approach is not, however, useful for applications which require tasks to be executed by the circuit before the power is activated. For example, in a circuit having a microcomputer which needs to store data in nonvolatile memory before the power is inactivated, this approach is not acceptable because it does not provide the microcomputer with the necessary time to store the data.
This problem has been recently addressed by employing a microcomputer to monitor the wake signals and to control the inactivation of the power. When the microcomputer recognizes that power should be inactivated, the microcomputer is able to first store the data and then disable the power supply. Although this implementation accommodates the microcomputer's need for storing such data before power is activated, it fails to provide fail safe operation.
Fail safe operation is needed to prevent the microcomputer from inadvertently inactivating the power supply. In an automotive application where the circuit controls the operation of the engine and/or dashboard indicators, such an inadvertent inactivation may severely impact the safety of the driver and others. In such applications, unless the microcomputer can effectively control the power supply in a fail safe manner, this approach is unacceptable.
Accordingly, a microcomputer power supply control circuit is needed which can operate in a fail-safe manner, yet overcome the above mentioned deficiencies.